How to completely remove an Animation Node tree from a blender file?
Open the node tree in the node editor.
While hovering over the node editor press space bar.
Search for Remove Animation Node Tree.
Execute the operation.


This should remove the tree completely.

I know it is a bit weird that you can't just delete the tree like others, the reason for it is a bit more technical. I don't want to go into details now.

The next version of AN will have a button in the footer/header of the node tree to remove the tree.